This invention relates to an air/fuel ratio control system for performing feedback control of the air/fuel ratio of an air/fuel mixture being supplied to an internal combustion engine, and more particularly to such a system which has an automatic control function of temporarily suspending supply of evaporated fuel from a canister to the intake pipe of the engine at low engine temperature operation and subsequently starting the supply of evaporated fuel to the intake pipe upon completion of warming-up of the engine.
An air/fuel ratio control system for performing feedback control of the air/fuel ratio of an air/fuel mixture being supplied to an internal combustion engine having an intake pipe has already been proposed by the assignee of the present application, which comprises means for detecting the concentration of an exhaust gas ingredient emitted from the engine, fuel quantity adjusting means for producing the mixture being supplied to the engine, and an electrical circuit operatively connecting the concentration detecting means with the fuel quantity adjusting means in a manner effecting the feedback control operation to control the air/fuel ratio of the mixture to a predetermined value in response to an output signal produced by the concentration detecting means.
An internal combustion engine provided with a carburetor in general has a canister which accommodates activated charcoal or a like material in which evaporated fuel is absorbed, the evaporated fuel being supplied from the float chamber of the carburetor and a fuel tank provided in the engine. This canister has its interior communicating with a venturi of the carburetor so that the fuel absorbed therein is purged into the venturi of the carburetor due to negative pressure produced in the intake pipe of the engine during operation of the engine.
At the start of the engine at low temperature, the quantity of intake air is restricted due to closing of a choke valve arranged upstream of the venturi so that the mixture in the intake pipe is very rich. If purging of evaporated fuel from the canister to the venturi is effected on this occasion, the mixture becomes too rich, which is disadvantageous to the operation of the engine. Therefore, a conventional carburetor engine is provided with a solenoid valve arranged in a control conduit which is arranged to introduce negative pressure (absolute pressure) in the intake pipe into the canister, and a special sensor which is used solely to this end and arranged to detect a low temperature condition of the engine. The solenoid valve is actuated in response to the output of the special sensor to cause temporary suspension of the fuel purging from the canister to the venturi at low engine temperature operation (purge cut).